The present invention relates generally to imaging systems. More particularly, it concerns thermal imaging methods and structures by which an image is formed using thermal energy.
Various types of imaging systems are known for forming a permanent image on an image carrier, the classic photochemical imaging system relying on photosensitive chemicals, such as silver halides, in granular form that capture incident photons to form catalytic sites on each exposed grain. During subsequent chemical processing, the exposed grains are reduced to their metallic form with a commensurate change in spectral absorption to define an image. In another type of imaging system, termed herein as a heat or thermally responsive system, the incident radiation on an imaging layer is utilized to effect a visible change by converting a portion of the incident radiation into thermal energy which affects one or more color-changing compounds or color-forming reactants to cause a visible color change or reaction, that is, a visible change in color or spectral absorptivity including a change from colorless to colored, from one color to another color, and from color to colorless. In addition to the reactants that participate in the color-forming reaction or other changes that alter spectral absorption, the imaging element can include absorber materials that absorb strongly at a selected wavelength, e.g., an infrared absorber, to promote the conversion of the incident radiation into thermal energy. As incident radiation passes into the imaging layer, the absorption of the radiant energy and its conversion into thermal energy falls off exponentially until the radiation, depending upon the depth of the imaging layer, is virtually fully absorbed, viz., extinguished. Accordingly, the temperature rise in the imaging layer is greatest on the side of the layer facing the radiation source and least on the opposite side. The incident radiation can take the form typical of a conventional photographic exposure, that is, as presented through imaging optics, or an exposure on a pixel-by-pixel basis, e.g., where a laser or other source illuminates, in a successive manner, selected spot-like picture elements on the imaging layer until the entire image is exposed.
The thermally responsive imaging elements have included the transfer type in which first and second reactants are provided in separate donor and receptor layers with the imagewise application of heat energy causing the reactants in the donor layer to inter-react with those of the receptor layer to provide the desired visible change. In addition to providing the reactants in separate donor and receptor layers, the donor materials can be provided in the respective donor layers that are isolated by an intermediate receptor layer. Upon thermal exposure, the donor materials transfer to the intermediate receptor layer to participate in the color-forming reaction. Also, the various materials may be contained in capsules which are ruptured upon exposure to thermal energy to cause the desired change in spectral absorptivity.
Known thermal imaging systems which employ first and second color-forming reactants or a single, independent color-changing compound as the color-imaging materials are disclosed in U.S. Pat. No. 4,392,141. Other color-changing compounds which undergo a visible change in color by the formation of color, the bleaching of color or by a color change in response to thermal energy are disclosed in U.S. Pat. Nos. 3,723,121, 3,745,009, 3,832,212, 4,380,629 and in copending and commonly assigned U.S. patent application Ser. No. 646,771, filed Sept. 4, 1984 by A. L. Borror et al.
There are a number of competing design considerations applicable when designing an imaging element of the type having a single thermally responsive imaging layer coated on or otherwise applied to a supporting substrate or carrier layer. Generally, the thickness of the imaging layer should be thick enough so that the incident radiation, as it penetrates into and is attenuated in the layer, is fully absorbed and converted to heat energy. Also, it is desirable for the incident radiation to develop no more thermal energy within an irradiated area in the imaging layer than necessary to promote the desired color-imaging reaction. Any energy in the irradiated area in excess of that required for the color-imaging reaction is wasteful, and the excess energy can be conducted to an adjacent area to cause, depending upon the energy transferred, color-changes in areas that are not directly irradiated. Where a multilayer imaging element is used, excess heat energy developed in one layer can also be conducted to an adjacent layer to also induce an undesired color-imaging reaction. In order to minimize undesired thermal conductivity effects, the time duration of the exposure radiation is preferably kept relatively short so that the desired energy is introduced into the imaging layer in a time frame that is less than the associated physical heat transfer times in order to minimize heat transfer from the irradiated to non-irradiated areas. Relatively short exposure intervals that introduce the desired quanta of energy can be achieved by increasing the radiant intensity of the exposure radiation. However, increased irradiance beyond certain upper limits can cause local overheating and overtemperature problems in the imaging layer on the side facing the radiation source, these problems include the formation of gasses and degradation of any polymers in the layer.